1. Field of the Invention
This invention relates to a protective circuit for a pulse width modulated signal amplifier, and more particularly to a protective circuit for a pulse width modulated signal amplifier having a simple construction and a reliable operation.
2. Description of the Prior Art
A pulse width modulated signal amplifiers using low frequency amplifiers with large outputs are known in the art. Transistors used in such amplifiers have the advantage that the efficiency of the system is superior since they perform on-off operations with a pulse width modulated carrier signal of frequency normally in a range of 100 KHz to 500 KHz, but they also have the drawback that they are susceptible to destruction by heat in comparison to prior art class B or class AB push-pull amplifiers.
A number of protective circuits have been proposed for class B or class AB push-pull amplifiers, but a protective circuit effective for use in pulse width modulated signal amplifiers has not yet been realized.
The prior art protective circuit for a pulse width modulated signal amplifier has the following problem.
A transistor used in the pulse width modulated signal amplifier is normally turned on and off with a modulated carrier signal, but when the modulated carrier is stopped by some cause, the transistor keeps its on-state for a time longer than that in the case of normal operation and hence there is the danger that the transistor will be destroyed instantaneously.